Paper type is an essential factor for any type of job being printed. However, its properties and characteristics are what determine the capabilities and limitations it has in order for it to be applicable for the appropriate job. Folding and paper strength are very important indications of the durability of archival papers.
What is Folding Endurance?
Once you have printed the job and folded it, you notice the finished product exhibits splitting at the spine. This may be the result of your paper undergoing stress and causing the fibres and coating to uplift. These types of problems can be addressed in a corrective manner by understanding the primary characteristics of paper which contribute to a paper’s folding endurance.
Folding endurance is measured through the paper’s ability to withstand multiple folds under a constant load prior to breaking. This is defined by several characteristics such as fibre strength, the fibers ability to effectively delaminate at the score, and sheet pliability. Fibre strength is dependent on the amount of fibres within a sheet. An increase in the percentage of fibres will strengthen the paper’s durability. This is further enhanced through moderate levels of refining, which is the interlacing of the bonds between the fibres. The paper pliability can be achieved through increasing the moisture content in order to enhance the effects of elasticizing the fibres. It is also important to take into consideration the grain direction to which the product is being folded; the endurance of the fold becomes greater against the grain because it performs a stiffer hold, which will less likely cause it stress if it were along the grain.
Tensile Strength
It’s defined as the maximum amount of tensile stress that is reached before breakage. The tensile strength of paper becomes more enhanced through the process of refining because of the greater degree of fibre bonding that occurs during paper-making. However, excessive refining can reverse the properties causing the paper to be weaker. By prolonging the refining stage, it can cause extreme fibre cutting when the paper reaches its peak value; the paper becomes more hard and less compressible.
As fibre length is increased, it furthers the paper’s ability to withstand breakage; the longer fibres tend to distribute the stress over more fibres and bond, while short fibres allow the stress to be concentrated in a smaller area. In addition, all papers have higher tensile strength in the machine or grain direction, allowing the fibre to fold more easily and making it ideal for greater resistance of being torn across the grain.
Bursting Strength
It is essential to understand that the strength of paper is not necessarily correlated with its thickness, but rather, the way the paper is formed. Bursting strength is characterized as an internal sheet property, defined by how much hydrostatic pressure a sheet can withstand before it ruptures. Such properties that affect bursting are fibres, beating, surface sizing, and the coating of the paper. Long fibres increase the strength of paper since it allows the fibres to intermingle and interlock into one another. However, the processing of the paper is what mostly contributes to the paper’s strength properties. Beating and refining increase the bursting strength (as well as other properties) of paper since it assists in the bonding of the fibres. The process encourages an increase in the flexibility of the fibre and internal and external fibrillation, which enhances its strength. Additives are also applied to the paper to define its characteristics. Surface sizing fill in voids and spaces among fibres and adds stiffness, which increases surface strength of uncoated paper.
Conclusion
The properties of paper from paper-making to printing are extremely important for the end-use application to determine the durability of paper for products such as wrapping, bags, and twisting. Such products can endure much more physical force which explains the necessary increased strength to withstand folding, tearing, and ripping.
